Recently, a photonic crystal has been drawing attention as a new optical device. The photonic crystal is a functional material having a periodical distribution of refractive index, which provides a band structure with respect to the optical and electromagnetic energies. Especially, the photonic crystal is characterized by the fact that it forms an energy region (a photonic band gap) therein through which neither light nor electromagnetic wave are impossible to propagate.
By introducing an appropriate defect in the distribution of refractive index in the photonic crystal, an energy level belong to the defect (defect level) is created in the photonic band gap. In this state, only light with the wavelength corresponding to the energy of the defect level can be present in the wavelength band corresponding to the energies in the photonic band gap. Defects arranged in a line in a photonic crystal constitute a waveguide, while a point-like defect in a photonic crystal works as a resonator.
A photonic crystal can be a two-dimensional crystal or a three-dimensional crystal. Though either of the crystals has respective advantages, the two-dimensional crystal is advantageous in that the crystal is comparatively easy to manufacture. In Japanese Unexamined Patent Application No. 2001-272555, it is described that, in a two-dimensional photonic crystal, a periodical distribution of refractive index is created by arranging cylindrical holes periodically in a triangular lattice pattern, that a waveguide is formed by rendering the cylindrical holes defective in a linear arrangement ([0025] and FIG. 1), and that a point defect is formed in the vicinity of the waveguide ([0029] and FIG. 1). Further, in Japanese Unexamined Patent Application No. 2001-272555, a point defect created by increasing the diameter of a cylindrical hole among those periodically arranged has been described as an example.
The applicants of the present application have proposed, in Japanese Unexamined Patent Application No. 2003-279764, to create a cluster defect by rendering defects in two or more adjoining modified refractive index areas among those constituting the periodical distribution of refractive index. A defect of the modified refractive index area is created by rendering the refractive index of a modified refractive index area different from that of other modified refractive index areas. In this construction, a defect of a modified refractive index area whose refractive index is lower than that of other modified refractive index areas is called an acceptor type defect, and a defect of a modified refractive index area whose refractive index is higher than that of other modified refractive index areas is called a donor type defect. A defect created by enlarging a cylindrical hole, which is described in Japanese Unexamined Patent Application No. 2001-272555, is an acceptor type defect, and a defect created by providing no modified refractive index area is a donor type defect. A cluster defect, and a point defect created by rendering only one modified refractive index area defective, are collectively referred to as “point-like defect”.
In Japanese Unexamined Patent Application No. 2003-279764 mentioned above, the applicants of the present application have proposed a two-dimensional photonic crystal with an in-plane heterostructure that has plural forbidden band zones each having modified refractive index areas with a different cycle distance from one another, where, in each of the zones, a point-like defect is created. With such a construction, when point-like defects in the same shape are provided in the respective forbidden band zones, lights with different wavelengths can be resonated at respective point-like defects due to differences in the cycle distance of the modified refractive index areas.
A variety of applications can be thought of the two-dimensional photonic crystal having such point-like defects. As a typical example, an optical multiplex communication can be shown. For the optical multiplex communication of recent years, the wavelength-division multiplexing scheme is adopted, in which lights with plural wavelengths each carrying respective signal are propagated along a single transmission line. A two-dimensional photonic crystal, by providing plural point-like defects corresponding to respective wavelengths in the vicinity of a waveguide, can be used as a demultiplexer for taking out lights (signals) with specific wavelengths out of lights propagating in the waveguide from the point-like defects, or alternatively, as a multiplexer for introducing lights with specific wavelengths into the waveguide from the point-like defects.
In the case where a conventional two-dimensional photonic crystal described above is used as a demultiplexer, the demultiplexing efficiency will be 100% if, among lights passing through the waveguide, all of light with the wavelength to be demultiplexed flows into the point-like defect. However, actually, more than 50% of the light with wavelength to be demultiplexed passes over the waveguide without flowing into the point-like defect. Hence, an actual demultiplexing efficiency is 50% or less.
In the case where the two-dimensional photonic crystal is used as a multiplexer, when the light to be multiplexed flows into the waveguide from a point-like defect, the light is divided into two ways of the waveguide. Therefore, the take-out efficiency of multiplexed light from the waveguide is 50% at the highest.
The present invention has been made in order to solve such problems, and it is an object of the present invention to provide a two-dimensional photonic crystal optical multiplexer/demultiplexer with high multiplexing efficiency and demultiplexing efficiency.